1. Field of the Invention
This invention relates to a method for forming a resist pattern and a photomask and, more particularly, to a method for forming a resist pattern used to manufacture a microscopic three-dimensional structure, and a photomask adapted for use in formation of the resist pattern.
2. Description of the Related Art
As is well known in the art, a lithography technique has been used in the micro-machining field for manufacture of semiconductor devices. In a general lithography technique, a planar shape of a photomask pattern is transferred into a resist pattern, and then the resist pattern is transferred onto a target substrate using an etching process to form a pattern similar to the photomask pattern on the target substrate.
In recent years, attempts have been made to etch a substrate in three dimensions using lithography techniques. For example, using an optical transmission system in which an optical signal from a semiconductor laser is propagated by an optical fiber and is received by a light receiving diode, high transmission efficiency has been achieved by focusing oscillating light from the semiconductor laser into a lens and by adjusting the spot diameter of the oscillating light at a fiber incident surface to approximate the core diameter of the optical fiber. An example of a conventional method of manufacturing a fine lens having a diameter of 250 μm or so (hereinafter referred to as a micro lens) used for such high transmission efficiency will be described in brief below (for example, see Japanese Patent Application Publication No. Hei8-166666 and Japanese Patent Application Publication No. 2003-177507).
A manufacturing method disclosed in Japanese Patent Application Publication No. Hei8-166666 has the following steps. First, a resist film is formed by coating a resist material on a target substrate on which micro lenses are to be formed. Next, the resist film is exposed using a photomask provided thereon with a plurality of concentrically formed light shielding films as light shielding regions on a transparent mask substrate, with spaces as light transmitting regions provided between the light shielding regions. Next, the exposed resist film is developed to form resist patterns. The formed resist patterns are thereafter transferred by dry etching onto the target substrate for forming the micro lenses.
In the photomask used therein, the spacing of the light shielding films in a direction along a diameter of a concentric circle is smaller than the resolution threshold of an optical system of an exposure device for which the photomask is used. Accordingly, light passing through the photomask has contrast insufficient to resolve photomask patterns. In addition, the width of the adjacent spaces (the spaces between light shielding films) gradually increases from the center of the concentric circles towards the outer diameter of the concentric circles.
On this account, the light intensity for exposure of the resist film gradually increases moving towards the outer diameter of the concentric circles. When a positive resist exposed using this photomask is developed, a resist pattern having a film thickness decreasing gradually toward the outer side along a diameter is formed.
In addition, Japanese Patent Application Publication No. 2003-177507 discloses a method of forming phase gratings as a photomask by a using plurality of grooves on a mask substrate and gradually changing the intensity of transmitted light with the depth or width of each of the grooves.
The photomask disclosed in Japanese Patent Application Publication No. Hei8-166666 utilizes concentrically arranged light shielding films. A resist film having a uniform thickness at an equal distance from a center (such as a spherical lens) can be easily formed by using photomask patterns having the concentrically arranged light shielding films. Otherwise, the photomask having the concentrically arranged light shielding films can not be used.
In the photomask disclosed in Japanese Patent Application Publication No. 2003-177507, it is difficult to precisely control the depth of each of the grooves constituting the phase gratings. Thus, there arises a problem of deviation in intensity of transmitted light and hence large deviations in resist pattern shape.
In addition, the resolution in a thickness direction of the resist film depends on the maximum and minimum space width in the photomask disclosed in Japanese Patent Application Publication No. Hei8-166666, and depends on the controllability of the depths of the grooves constituting the phase gratings in the photomask disclosed in Japanese Patent Application Publication No. 2003-177507. The resolution in a horizontal direction of the resist film depends on the arrangement interval of the mask patterns in the photomask disclosed in Japanese Patent Application Publication No. Hei8-166666 and the photomask disclosed in Japanese Patent Application Publication No. 2003-177507.
There is a need to decrease the minimum space width of the photomask to increase the positional precision in the horizontal direction or film thickness direction of the resist film, but this may result in significantly increased costs for manufacturing of the photomask. As a result, there is a limit to raising of the positional precision of the resist patterns with the mask patterns alone.